The reinforcement armature or reinforcement of tires and in particular of tires for agricultural vehicle is currently—and most frequently—formed by stacking one or more plies conventionally referred to as “carcass plies”, “crown plies”, etc. This manner of designating the reinforcement armatures is derived from the manufacturing process, which consists of producing a series of semi-finished products in the form of plies, provided with cord reinforcing threads which are frequently longitudinal, which products are then assembled or stacked in order to build a tire blank. The plies are produced flat, with large dimensions, and are subsequently cut according to the dimensions of a given product. The plies are also assembled, in a first phase, substantially flat. The blank thus produced is then shaped to adopt the toroidal profile typical of tires. The semi-finished products referred to as “finishing” products are then applied to the blank, to obtain a product ready to be vulcanized.
Such a type of “conventional” process involves, in particular for the phase of manufacture of the blank of the tire, the use of an anchoring element (generally a bead wire), used for anchoring or holding the carcass reinforcement in the zone of the beads of the tire. Thus, in this type of process, a portion of all the plies constituting the carcass reinforcement (or only a part thereof) is turned up around a bead wire arranged in the tire bead. In this manner, the carcass reinforcement is anchored in the bead.
The general adoption of this type of conventional process in the industry, despite the numerous different ways of producing the plies and assemblies, has led the person skilled in the art to use a vocabulary which reflects the process; hence the generally accepted terminology, comprising in particular the terms “plies”, “carcass”, “bead wire”, “shaping” to designate the change from a flat profile to a toroidal profile, etc.
There are nowadays tires which do not, properly speaking, comprise “plies” or “bead wires” in accordance with the preceding definitions. For example, document EP 0 582 196 describes tires manufactured without the aid of semi-finished products in the form of plies. For example, the reinforcement elements of the different reinforcement structures are applied directly to the adjacent layers of rubber mixes, the whole being applied in successive layers to a toroidal core the form of which makes it possible to obtain directly a profile similar to the final profile of the tire being manufactured. Thus, in this case, there are no longer any “semi-finished products”, nor “plies”, nor “bead wires”. The base products, such as the rubber mixes and the reinforcement elements in the form of cords or filaments, are applied directly to the core. As this core is of toroidal form, the blank no longer has to be shaped in order to change from a flat profile to a profile in the form of a torus.
Furthermore, the tires described in this document do not have the “conventional” upturn of the carcass ply around a bead wire. This type of anchoring is replaced by an arrangement in which circumferential cords are arranged adjacent to said sidewall reinforcement structure, the whole being embedded in an anchoring or bonding rubber mix.
There are also processes for assembly on a toroidal core using semi-finished products specially adapted for quick, effective and simple laying on a central core. Finally, it is also possible to use a mixture comprising at the same time certain semi-finished products to produce certain architectural aspects (such as plies, bead wires, etc.), whereas others are produced from the direct application of mixes and/or reinforcement elements.
In the present document, in order to take into account recent technological developments both in the field of manufacture and in the design of products, the conventional terms such as “plies”, “bead wires”, etc., are advantageously replaced by neutral terms or terms which are independent of the type of process used. Thus, the term “carcass-type reinforcing thread” or “sidewall reinforcing thread” is valid as a designation for the reinforcement elements of a carcass ply in the conventional process, and the corresponding reinforcement elements, generally applied at the level of the sidewalls, of a tire produced in accordance with a process without semi-finished products. The term “anchoring zone”, for its part, may equally well designate the “traditional” upturn of a carcass ply around a bead wire of a conventional process and the assembly formed by the circumferential reinforcement elements, the rubber mix and the adjacent sidewall reinforcement portions of a bottom zone produced with a process using application on a toroidal core.
With regard to the usual design of tires for agricultural vehicle, the carcass reinforcement, anchored within each bead, is composed of at least one layer of textile and/or metallic reinforcement elements, said elements being substantially parallel to each other in the layer and possibly being substantially radial and/or clearly crossed from one ply to the next, forming equal or unequal angles with the circumferential direction. The carcass reinforcement is usually surmounted by a crown reinforcement composed of at least two working crown layers of reinforcement elements which may be textile or metallic ones, but which are crossed from one layer to the next, forming small angles with the circumferential direction. The tread of the tire in question is formed of blocks of rubber or bars, inclined relative to the circumferential direction by a generally large angle, and separated circumferentially from each other by hollows having a width (measured in the circumferential direction) greater than the average width of the bars. Said bars may be symmetrical to each other relative to the equatorial plane, being axially continuous or, as in the majority of cases, axially discontinuous. The ends of bars axially close to the equatorial plane are then in the majority of cases offset circumferentially relative to one another, while having what is commonly called a chevron design.
Tires for agricultural vehicle, such as described above, are usually subject to a pressure of between 1.1 and 1.4 bar in order to use the vehicle on cultivated land for conventional loads and dimensions, corresponding to a maximum amount of deflection of the tire of less than 28%.
The maximum amount of deflection is defined as being the maximum deflection divided by the height of the sidewall H, which is defined later.
The deflection of a tire is defined by the radial deformation of the tire, or variation in radial height, when it changes from a non-loaded state to a statically loaded state, under rated load and pressure conditions.
When this same agricultural vehicle has to travel on hard ground or on roads, at higher speeds, it is necessary to have higher pressures, which may be as much as 2 bar, in order to retain satisfactory endurance of these tires and prevent excessively fast wear.
To perform these changes of pressure, it is necessary to have a device such as a compressor or a pressurized air reserve, generally on board the machine, the latter by definition being on the move when it becomes necessary to increase the pressure of the tires.
The major drawback of these devices, in addition to their presence, their cost and their maintenance, is the time necessary to change the pressure of the tires.
Furthermore, one current requirement of users is to reduce still further the risk of damage to crops during use on cultivated land due to the crushing of said crops when the vehicles, and more specifically the tires, pass over them.
The large amount of research carried out by the Applicant relating to modifications of meridian profiles of treads, of meridian profiles of carcass reinforcements, of the material of the carcass and crown reinforcements, modifications of design and dimensions of tread-pattern blocks has not, to date, provided the expected improvements.
It is furthermore known to use dual tires, which may make it possible to limit the crushing by reducing the pressure of each of the tires while maintaining the loading capacity. However, such solutions lead to problems of bulk in particular on roads.